1. Field of the Invention
The present invention relates to anchors for supporting an object on a supporting substrate. A specific example of such anchors includes the field of dental implants of the type permanently installed into a patient""s bone, such as a jaw bone. More particularly, the invention improves retention of an anchor by configuring grooves or channels cut into external threads of the anchor such that spontaneous counter-rotation or unthreading of the anchor is opposed. The invention is also usable in the form of industrial fasteners. The invention finds utility wherever anchors are threaded into a solid supporting object for long term installation. It is contemplated that the primary applications will be those of dental and other medical anchors or implants, and associated prostheses.
2. Description of the Prior Art
In the field of dental prostheses, a trend has developed favoring anchoring of prostheses in the jaw bones of patients. Particularly as biocompatible materials have become available, it is possible to contemplate long term successful installations of prostheses. The process of implantation begins with placement of the anchoring element, or implant, within the jaw. Most implants are threaded into a hole bored into the jaw, or are pressed into a similar hole. After the implant has become osseointegrated, the prosthesis, mounted on a suitable post, is fixed to the implant in a subsequent step.
It will be appreciated that natural teeth and prostheses functioning in a similar manner are subjected to great forces, such as repeated compression and recovery cycles. It is possible for the implant to unthread spontaneously and become loose. Implants are quite small, and can be swallowed or otherwise lost. Even if they are retained, looseness can cause injury and subsequent loss if chewing and biting are attempted.
The prior art has recognized the problem of unintended detachment of the implant. U.S. Pat. No. 4,826,434, issued to Kenneth K. Krueger on May 2, 1989, shows a dental implant having axial grooves cut through threads at the distal end of the implant. Each of these grooves has a first face arranged perpendicularly or nearly so to the outer circumferential surface of the implant, and a second face oriented at an angle such that the second face is closer to parallel to the circumferential surface than is the first face.
However, in Krueger, the grooves are oriented in a manner typical of self-tapping screws, that is, the perpendicular face is exposed in the direction of motion during threading. By contrast, this orientation is reversed in the present invention. While any irregularity formed in threads will oppose unthreading due to increased surface area of osteointegration, the orientation of the present invention acts more in ratchet fashion, thereby more effectively opposing unthreading.
Also, in Krueger, these grooves are located at the distal end of the implant. By contrast, corresponding grooves in the present invention extend for longer along the length of the threaded section of the implant. In Krueger, the grooves are quite short, and cut entirely through the threads and extend into the shank.
U.S. Pat. No. 4,932,868, issued to Leonard I. Linkow et al. on Jun. 12, 1990, illustrates an implant having axially oriented grooves cut through the threads. Each groove has one face arranged nearly perpendicularly to the outer circumferential surface of the implant, and a second face oriented at an angle much closer to parallel to the outer circumferential surface. By contrast with Linkow et al., the present invention reverses the relative positions of the two faces of each groove. The novel arrangement forgoes bone shaving asserted to occur by Linkow et al., but gains instead ability to resist spontaneous counter-rotation or unthreading of the implant.
U.S. Pat. No. 4,863,383, issued to Hans L. Grafelmann on Sep. 5, 1989, illustrates a dental implant having axially oriented grooves for self-tapping purposes. The grooves are similar to those of the device of Linkow et al., and share structural dissimilarities distinguishing the present invention from those of Krueger, Linkow et al. and Grafelmann.
My prior U.S. Pat. No. 5,601,429, issued on Feb. 11, 1997, describes a dental implant having relief grooves formed transversely in exterior threads. The threads have surface pitches disposed to oppose unthreading. However, the grooves of the prior issued patent do not have characteristics of the present invention.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.
The present invention provides an anchor which has structure opposing spontaneous withdrawal or loosening. As described herein, the invention will be described in terms of a threaded dental implant, it being understood that the inventive concept is equally applicable to other anchors and fasteners wherein it is desired to oppose unthreading. The threads of the implant are interrupted by axially or longitudinally oriented grooves or channels. Each groove has a first face oriented perpendicularly or nearly so to the circumferential outer surface of the implant, and a second face which assumes an orientation much closer to parallel to the circumferential surface than is the first face.
The first face is located so that it does not oppose threading of the implant into its associated hole. However, because of resiliency of bone tissue, reversal of the direction of rotation in which the implant is installed will cause interference as the retentive element snags bone tissue. Therefore, the second face acts in the manner of a ratchet allowing one direction of rotation yet opposing the opposite direction.
The grooves are formed above the distal end of the implant, where the distal end is that first contacting the hole and bone tissue when the implant is installed. The grooves extend along a tapered or generally frustoconical section of the implant almost to the socket portion, where the socket portion is that portion which is cylindrical at its outer surface and which bears structure for engaging the post of the prosthesis. The grooves therefore extend along the implant for a significant distance of the threaded portion thereof. The undisturbed threads serve as starting threads. In some embodiments, the grooves do not cut entirely through the threads and therefore do not intrude upon the frustoconical shank bearing the threads. In other embodiments, the threads are sufficiently deep to intrude upon the shank.
In the first embodiment, bone tissue is displaced radially both when being compressed and when expanding. In an alternative embodiment of the invention, the grooves are replaced by projections generally configured as arrowheads having leading and trailing faces arranged to displace bone tissue axially with respect to the implant. As each projection passes, bone tissue expands resiliently in an axial direction opposite to that in which it was compressed. The trailing face in the second embodiment is arranged perpendicularly or nearly so to the axis of rotation of the implant, whereas the leading face is arranged at a much less severe angle. The second embodiment therefore opposes rotation in a manner similar to that of the first embodiment, the difference being direction of displacement of bone tissue.
The improvement provides more immediate retention of an implant in bone tissue. The constituent material is biocompatible, titanium for example, and therefore contributes to the osteointegration process.
Implants of different configurations can enjoy the benefits of the invention. The socket may bear threaded and unthreaded cylindrical recesses for receiving the post of a prosthesis. The threaded shank may be tapered or cylindrical, and may be equal in diameter to the socket or alternatively may be smaller than the socket.
Accordingly, it is one object of the invention to provide an anchor which resists spontaneous unthreading from its anchorage in bone tissue.
It is another object of the invention to promote long service life of implanted dental and other prostheses supported in a live body by osteointegration.
It is a further object of the invention that the invention be applied to anchors having sockets and threaded shanks of different dimensions and configurations.
It is an object of the invention to provide improved elements and arrangements thereof in an apparatus for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.